1. Field of the Invention
The invention relates generally to radio communication and more specifically to navigation receivers that operate with signals received from orbiting navigation satellites and equipment and methods for reducing multipath signal interference.
2. Description of the Prior Art
The global positioning system (GPS) supported by the United States government enables satellite navigation for military and civilian users alike. Two codes, a coarse-grained acquisition code (C/A-code) and a fine-grained precision code (P-code) are transmitted on two L-band microwave frequencies, e.g., L1 on 1575.42 MHz and L2 on 1227.6 MHz, and each provide ranging and GPS-system time information. The C/A-code is available to civilian users and the P-code is available only to authorized users. During certain periods, the P-code is encrypted and such encrypted P-code is referred to as Y-code. The C/A-code is a digital sequence that repeats each millisecond and is unique to one of two dozen satellites. The P-code is a digital sequence that has a period of 269 days, with one week long segments of it transmitted intact. A single week-long segment is 10.23.times.10.sup.6 .times.604800 bits long, which comes from the P-code transmission rate of 10.23 MHz times the exact number of seconds in seven whole days. So a code phase uncertainty of even plus-or-minus one second can call for a search through 20,460,000 chips.
Although each orbiting GPS satellite may transmit a single carrier for each frequency and only one receiver antenna is needed to receive that carrier signal, the direct signal and many other signals that took longer paths to the receiver due to reflections can be simultaneously received. The phases of the received multipath signals can combine to add or subtract from the direct path carrier signal and cause phase distortions. Because the multipath signals take a longer path, they always arrive at the receiver later than the direct path carrier signal. Conventional GPS receivers resolve their positions to accuracies of plus-or-minus two centimeters by measuring the carrier phase of either or both of the L1 and L2 carriers which have respective wavelengths of nineteen centimeters and twenty-four centimeters.
The carrier signal is spread spectrum modulated with a psuedorandom code sequence and conventional GPS receivers lock on to both the carrier phase and the code phase. The spread spectrum modulated carrier signal is despread by correlating the downconverted signal with a local code. An autocorrelation maximum will occur when the local code phase exactly matches the received code phase. Proper carrier phase locking depends on proper code phase locking. The code comprises 1023 chips and as the local code phase is slipped plus-or-minus one-half chip, the output power of the correlator will rise from zero through maximum at the exact local code phase match and back to zero, e.g., in a triangular waveform. Early, punctual and late versions of the local code phase are used to characterize the carrier phase lock. The presence of multipath carrier signals will distort the late side of the autocorrelation waveform and thereby cause the receiver to lock on to a local code phase that is later than the ideal phase. In such a case, the carrier phase lock will also be in error. Because multipath carrier signal energies are so erratic, the point of local code phase lock can similarly be erratic.
The GPS carrier phase measurement is subject to multipath errors generated from one or more reflected signals entering the antenna. There would be no carrier phase multipath if the antenna connected to the receiver was so directional it could only receive the direct signal from each satellite, assuming no secondary reflected signals from the satellites themselves. In practice, the receiver antenna is omnidirectional subject to one or more reflected signals as well as the desired direct signal. The magnitude and phase of the reflected signals with respect to the direct signal is dependent on the environment the antenna is operating in and thus is subject to considerable change, especially in applications where the antenna is moving, e.g., in navigation, differential and real-time kinematic modes.
The period and magnitude of the multipath is dependent on the electrically-reflective environment the antenna is situated in. The carrier phase multipath error is cyclical in nature and can be reduced by measurement averaging. But averaging requires ten to fifteen minutes of holding stationary to average the collected measurements to significantly subtract out the carrier phase multipath induced errors. In many applications, the antenna cannot be held still and it is therefore subject to significant variations in the multipath signature and a random loss of satellite lock. So in practical terms, averaging is not a solution for significantly reducing carrier phase multipath. Other conventional attempts to mitigate multipath have included altering the antenna gain pattern such that signal reflections from low elevation, ground level, objects are reduced. These techniques usually require physically large, non-portable antenna designs, e.g., choke ring antenna. Even so, high-elevation multipath sources would not be screened out and could still cause distortion.
The carrier tracking loop in conventional GPS receivers conventionally control the phase difference between incoming and locally generated carriers to approach zero, thus driving the signal magnitude observed in the conventional quadrature correlators to zero. When the carrier tracking loop is closed for the ideal non-multipath case the direct signal vector (S.sub.D) is inphase (0.degree.) with the observed signal (S.sub.O). Although it is the observed signal that the carrier tracking loop locks to, the lock will have zero phase error to the direct signal, thus no problem arises. But in the presence of a multipath signal (S.sub.M), the direct signal and the multipath signal (S.sub.M) vectors will combine to form the vector product which is accepted by the receiver as the observed signal. In this case, the vector of the multipath signal induces a false measurement of both the amplitude and phase of the direct signal, and the carrier tracking loop will produce an inphase signal (I) that is not zero degrees apart from the actual direct signal. The ninety-degree (90.degree.) quadrature signal (Q) will similarly be affected. Such phase errors will be interpreted by the receiver as placing the ultimate position estimate of the receiver away from its true location. The magnitude and direction of the displacement depends on the signal strength and path delays of the multipath signals.